In an analog output function of a remote unit for a programmable logic controller in a distributed control system, in order to change or stop an analog output of the remote unit by inputting a state of a control target (digital signal, analog input, etc. from sensor) to the remote unit and determining an abnormality of the control target from the state externally inputted, it has been necessary that a CPU unit of the programmable logic controller executes a control program such as a ladder program to monitor the state of the control target.
Therefore, a determining time for the CPU unit detecting the abnormality of the control target from the external input of the remote unit and output updating time for changing the analog output of the remote unit depend on the operating cycle of the CPU unit and the communication cycle between the CPU unit and the remote unit, and thus update of the analog output has been delayed owing to variations in the operating cycle and the communication cycle. Further, in a case where execution of the control program is stopped owing to failure of the CPU unit, it has been unable to detect the abnormality of the control target from the external input of the remote unit and then to change the analog output of the remote unit.
With regard to the problem, a technique has been proposed in which operation of the arithmetic unit in the programmable logic controller is monitored and the analog output is forcibly changed when failure of an arithmetic unit in the programmable logic controller is detected (for example, refer to Patent Document 1).